Effect of Mn and Si Addition on Melt Pool Oxidation of AISI316L during Selective Laser Melting

Abstract

SLM(Selective Laser Melting)타입의 적층 주조 공정에서 공정변수와 보조가스의 유량 및 AISI316L 파우더의 조성 비를 다르게 하였을 때 나타나는 산화 개재물의 성장 거동 및 물성에 대해 평가해보았다. 조성비의 변화는 기본 AISI316L 조성을 기준으로 실리콘 1배, 2배, 망간1배를 추가함 으로 제작한 파우더이며 균질한 조합을 위해 mechanical mixing 방법을 채택하였다. 각각 따른 NO분석과CS 분석, 인장 시험, 광학 현미경 및 SEM전자현미경을 통한 개재물의 분산 정도 및 강도의 차이와 공극률 및 체적에너지밀도의 차이를 확인 할 수 있었다. 챔버 내부의 산소분위기에 의한 차이는 미미했으나 이는 보조 가스의 영향에 대해 알 수 있는 지표로써 작용하였다. 실험 결과로는 VED가 산소 농도에 영향을 미치는 이유는 용탕에 머무르는 빔의 세기 및 스캔 속도에 관련이 있고 조성 별 로 차이를 미치는 이유는 엘링햄 다이어그램의 산소 친화도와 관련이 있다고 해석된다. SLM 공정중 일어나는 산화와 환원반응을 Vapor Jet Domain Model [Figure 6.1.4] 로 설명하였으며, CO가스와 직접적으로 일으키는 산화 환원, 조성적차이에 의한 CO 가스와 간접적으로 일으키는 산화 반응식에 대해 살펴보았다. 공극률에 대한 차이는 공정 도중 생성되는 산화막의 젖음성(wetting behavior)와 관련이 있고 내부 공극의 갇힘 형상으로 설명하였다. 물성 평가 부분에 의한 효과의 기대는 미미했으나 연신율이 급격히 감소 하는 조건을 찾아냈으며, PDAS 값과 개재물의 분산 정도에 대해 알아보았고 2Si 와 같은 Failure Mode 도 찾아낼 수 있었다. 오로완 메커니즘에 기반한 산화개재물의 분산강화효과에 대해 평가해보았고 망간의 추가의 특이점에 대한 부분을 알 수 있었다. SLM에서의 주요 강화 효과는 셀 벽 생성과 오로완 분산 강화 효과 임을 또한 찾아볼 수 있었다.

316L Stainless steel is the most abundant austenitic stainless steel that primarily constituents after iron, and chromium (~16%) nickel and molybdenum with small quantities of silicon and manganese. Selective Laser Melting (SLM) is known for the highest cooling rate (10^6)(K/s) among all additive manufacturing techniques. Differently with Laser Melting Deposition (LMD), its’ microstructural inclusion size is expected as small around a single-digit nanometer due to its high cooling rate. Heat treatment as post-processing of a solidification structure is unavoidable. The recent tendency of studying additive manufacturing is commonly about processing parameter control such as laser power and scan speeds that affect total volumetric energy density. Nevertheless, controlling inclusions by solidification structure are not have been studied further yet. Hence, the purpose of this thesis is majorly focused on the evaluation of mechanical properties and microstructural evaluations of the printed specimens under certain oxidation level, and chemical compositions’ ratio differences of 316L stainless steel powder. Preliminary Test has been performed to obtain a relationship between processing parameters (windows) and volumetric energy density which affects porosity of the specimen tend to achieve the ideal condition of the specimen which has less than 0.1% of porosity Controlling Oxidation Testing was performed with a closed system and non-closed system result experiment demonstrated certain properties and results, which have been expected by changing oxygen partial pressure, movement of laser beam and chemical composition during SLM process. Due to the unavoidable oxidation phenomena during additive manufacturing (AM) process, to understand oxygen consumption and melt pool oxidation, two sets of experiments were conducted using powder-bed fusion type AM machine: closed system and open system in terms of oxygen. In the closed system experiment, oxygen consumption was found to be linearly proportional to oxygen partial pressure in the chamber. Oxygen level decrease from 0.5 % to ~0.01 % during the AM process. The results indicated the amount of oxygen consumed by spatter and metal vapor fume was larger than that of oxygen consumed by melt pool oxidation. In the open system experiment, AM process was conducted under the three different Oxygen partial pressure levels. Oxygen contents in the samples showed a little difference between the samples, but the amount of spatter obviously increased as oxygen partial pressure increased. Changing Silicon and Manganese ratio could vary types of oxide inclusions within volume of specimen and distinguished differences between oxide inclusions and could affect dendritic arm’s chemical composition. The aim of this section is based on analysis of microstructural details by various types of Silicon and Manganese ratio, which have produced by selective laser melting technique. Property test has been evaluated through controlling three different sectors: compositional, shielding gas flow ratio, and processing parameters, which is the most significant research in the paper. Hence the melt pool oxidation and reduction in SLM were explained by Domain Model which lead both Oxygen and Nitrogen analysis. Both oxidation and reduction were controllable by the addition of deoxidizer elements. Porosity inside of the specimen was able to change by compositional differences also due to the oxide film’s wetting behavior. Orowan strengthening effect evaluated mechanical property of alloy addition. Failure mode with strong silicon addition was determined by the experiment and revealed that the Orowan mechanism worked slightly on a logarithmic scale. Inclusion number density could tell the influences of silicon addition and enabled to measure primary dendritic arm spacing that is directly related to the cooling rate of the material. The obtained results are interpreted with EBSD, EDS, Vickers Indentation, Uniaxial tension test, and NO analyzer.